Have you ever had two surfaces that you needed to combine together but the problem is, at the boundary of the inner surface, its elevations don’t match the elevations of the outer surface. In cases like this, if you paste them together you can get some really odd things going on where they are supposed to meet.

An example of this might be that you have one surface that was created from USGS data and another surface that was surveyed. They should be close to the same elevations but they won’t be exact. I often have people ask me if there’s a way to combine them but use a buffer between. Using a buffer you won’t get those almost vertical triangles or triangles that go out for quite a while until they connect into the other surface.

Pasting Surfaces Issue

In the above image, I have two surfaces, one with green triangles and a yellow border and one with grey contours. I need to paste them together to create a combined surface. Any surface points that are under the border of the inner surface when it’s pasted in will be removed and that white, thick line represents the triangles from the outer surface that are unchanged. As you can see, there are some odd things going on.

Since there’s no way to add a buffer when pasting surfaces, what do you do? Well, here’s how you do it in five simple steps:

Extract the border of the inner surface.

Offset this extracted border the buffer distance.

Assign the elevations from the outer surface to this new object.

Create a surface from this offset.

Paste all three surfaces together.

1. Extract the border of the inner surface.

You probably already know how to do this but, in case you don’t, it’s pretty simple. Just follow these steps:

Make sure the surface you want to extract the border from is using a style that actually displays the border (you can’t extract something if the surface isn’t dislaying it).

Select the surface and on the ribbon expand out “Extract from Surface” and choose “Extract Objects”.

Extract Objects

Select “Border” from the options in the next dialog box (deselect anything else you don’t want to extract from your surface) and click OK.

Select the Border to Extract

You now have a 3D polyline in your drawing where the border of the surface is.

2. Offset this extracted border the buffer distance

Again, pretty simple but I’ll explain the steps here. On the Modify tab of the ribbon, on Edit Geometry panel, there’s a command called, “Stepped Offset”.

Stepped Offset

Follow the command line prompts and offset it the distance you need. When it comes to setting the elevation, it really doesn’t matter what you choose as we’ll set the elevation of this new polyline in the next step. The AutoCAD Offset command most likely will not work as this is likely to be a 3D Polyline and the Offset command only works on 2D objects.

Offset Polyline

3. Assign the elevations from the outer surface to this new object

This new polyline needs the elevations of the outer surface. Still on the Modify tab of the Ribbon, on the Edit Elevations panel, there is a command called, “Elevations from Surface”.

Elevations from Surface

Run this command and select the polyline. Next you’ll see a new dialog box asking you which surface to use. Select the outer surface (in this example it’s called “Pre-EG”) and make sure you toggle ON the option for, “Insert intermediate grade break points”.

Elevations from Surface Options

Your new 3D polyline now follows the outer surface exactly and we’re ready for the next step.

4. Create a surface from this object.

Again, pretty simple but here are the steps.

On the Home tab of the ribbon, on the Create Ground Data panel, expand out Surfaces, and select the first option, “Create Surface”.

Name it appropriately (I would call it something like “<inner surface name> Pasting Buffer”. Set any other settings you want (the style really doesn’t matter – I would probably choose something like, “No Display” if it’s an option).

On the prospector, expand out the new surface, expand out the definition, right click on Breaklines and chose, “Add”. Select the offset 3d polyline and apply the breakline settings as desired.

And that’s it. You’re done.

5. Paste all three surfaces together.

Now, that you have done all that, we are ready to paste them all together. You can past them into the original outside surface but I’m not a fan of that. I would much rather have the outside surface remain intact in case I need to use it for something else. I typically will create a new surface (see step four for the steps to create a new surface).

On the Prospector tab, expand out the new surface, expand out definitions, and choose “Paste”. Select the surfaces you want to paste in. The order you paste them in is very important as whatever is within the border of the incoming surface will completely overwrite everything inside it. The order we will use here is 1) Outside surface 2) Buffer surface 3) Inner surface.

Paste Order

The following sequence of images show the progression of the new surface as the other three surfaces are pasted in. I left in the thick white line from earlier as a reference.

Outer Surface Pasted In

Buffer Surface Pasted In

Inner Surface Pasted In

As you can see, that buffer works very nicely. You can compare this to a surface that only has the outer and inner surfaces pasted in.

Same surface without the buffer surface

What do you think? Is this something you might use? Leave a comment if you do this a different way. I always love to hear about different ways of accomplishing things!

The data set I used is from the training manual “A Practical Guide to Civil 3D 2017” by Rick Elis. You can order a copy from his company CADapult if you would like one. This is the book I use in my classes.